Electronic circuits tend to be very sensitive to parasitic capacitances, especially those using programmable transconductance input stages. For example, programmable electronic filters such as transconductance/capacitance continuous-time filters are very sensitive to parasitic capacitances. The parasitic capacitances harm the performance of the filter by moving its poles and cut-off frequency through alteration of the overall input capacitance. Additionally, the accuracy of the amplitude of the frequency-response characteristic may be altered. In cases where the parasitic capacitances do vary, the frequency-response characteristic may be significantly altered.
Programmable electronic filters such as transconductance/capacitance continuous-time programmable filters have the ability to be programmed over a range of cut-off frequencies. The programming may be achieved by switching transconductance devices in or out of the input transconductance stage, depending on the desired cut-off frequency and frequency-response characteristic. This may be done by adding transconductance devices or removing them. For example, transistors, acting as transconductance devices, may be switched in or out to alter the overall transconductance of a circuit. Unfortunately, the adding and removing of transconductance devices introduces parasitic capacitances to the input. As discussed above, parasitic capacitances may significantly degrade or alter the performance of an electronic device by changing the overall input capacitance.
An electronic filter designed to provide a frequency-response characteristic having a single cut-off frequency, may be designed to account for any known parasitic capacitances. However, programmable electronic filters have the ability to provide a range of cut-off frequencies and this programming is usually achieved by adding or removing transconductance elements or devices. Such programming changes the parasitic capacitance and prevents the design of a circuit with a known capacitance to offset a known parasitic capacitance.